Spin Crossover and Exchange Effects on Oxygen Evolution Reaction Catalyzed by Bimetallic Metal Organic Frameworks

Bimetallic metal–organic frameworks (BMOFs) have
shown a
superior oxygen evolution reaction (OER) performance, attributed to
the synergistic effects of dual metal sites. However, the significant
role of these dual-metal synergies in the OER is not yet fully understood.
In this study, we employed density functional theory to systematically
investigate the OER performance of NiAl- and NiFe-based BMOFs by examining
all possible spin states of each intermediate across diverse external
potentials and pH environments. We found that the spin state featuring
a shallow hole trap state and Ni ions with a higher oxidation state
serve as strong oxidizing agents, promoting the OER. An external potential-induced
spin crossover was observed in each intermediate, resulting in significant
changes in the overall reaction and activation energies due to altered
energy levels. Combining the constant potential method and the electrochemical
nudged elastic band method, we mapped the minimum free energy barriers
of the OER under varied external potential and pH by considering the
spin crossover effect for both NiAl and NiFe BMOFs. The results showed
that NiFe exhibits better OER thermodynamics and kinetics, which is
in good agreement with experimentally measured OER polarization curves
and Tafel plots. Moreover, we found that the improved OER kinetics
of NiFe not only is attributed to lower barriers but also is a result
of improved electrical conductivity arising from the synergistic effects
of Ni–Fe dual-metal sites. Specifically, replacing the second
metal Al with Fe leads to two significant outcomes: a reduction in
both the band gap and the effective hole mass compared to NiAl, and
the initiation of super- and double-exchange interactions within the
Ni–F–Fe chain, thereby enhancing electron transfer and
hopping and leading to the improved OER kinetics.